Method and arrangement in connection with optical bragg-reflectors

ABSTRACT

A method of establishing transmission of light through a chirped Bragg-reflector, a method of analyzing the power spectrum of a light signal using a chirped Bragg-reflector, and an arrangement for analyzing the power spectrum of a light signal. The Bragg-reflector reflects, in an unperturbed state, essentially all incident light within a predefined wavelength range. The methods include the steps of directing the light to be analyzed into an input end of a light-guiding structure, such as an optical fiber, which light-guiding structure is provided with a Bragg-reflector, and sending an acoustic pulse along the light-guiding structure, thereby effectively lowering the reflectance of the Bragg-reflector for a certain wavelength at a certain time. By monitoring the light thus transmitted through the Bragg-reflector, a power spectrum analysis of the incident light is obtained.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This is a continuation of application Ser. No. 09/606,084, filedJun. 29, 2000, the disclosure of which is hereby incorporated byreference.

TECHNICAL FIELD OF THE INVENTION

[0002] The present invention relates to optical Bragg-reflectors, andmore particularly to the alteration of the reflective properties ofoptical Bragg-reflectors.

BACKGROUND OF THE INVENTION

[0003] The need for capacity in communications networks is increasingexponentially, and presently capacity demands are doubling everyeighteen months or even less. Efforts are being made in the telecom andcomputer business to meet this incredible expansion of bandwidthrequirements.

[0004] One obvious action is to simply draw new optical cables betweennodes of the network. However, this approach is costly and hardly asuccessful way of meeting the upcoming demands. Instead, measures arefocused, when possible, on augmenting the capacity of existing fibernetworks.

[0005] One widespread method of achieving increased transfer capacity inexisting fiber networks is wavelength division multiplexing (WDM). InWDM, a single optical fiber is used for the transmission of severalchannels of information, each channel being associated with a specificwavelength. For a background of this technology, the reader is directedto, “A review of WDM technology and applications”, Opt. Fiber Technol.,5, pp. 3-39 (1999).

[0006] While the use of WDM in optical networks yields a substantialincrease in transfer capacity, the complexity of the communicationssystems increases correspondingly. Furthermore, the number of physicalcomponents in each link of the network increases, the requirements oneach component increase, and detecting and localizing errors orimperfections in the network is rendered more difficult.

[0007] The optical networks of today mainly use point-to-pointcommunication, wherein electronically encoded information in one node ofthe network is transformed into optically encoded information, and thentransferred, through a sequence of optical fiber amplifiers and opticaltransport fibers, to another node, where the optically encodedinformation is transformed back into electrically encoded information.

[0008] However, this prior art is associated with major drawbacks andlimitations.

[0009] If, for instance, an error occurs in one of the fiber amplifiers,it is very hard to determine which amplifier, in the sequence ofamplifiers, is the faulty one.

[0010] Furthermore, technology is being pushed towards communicationssystems in which the optical signal is passed via all-optical switchingnodes, where the signal is routed without any need foroptical-to-electrical conversion or vice versa. Thus, if an error isdetected in a receiving node of the communications network, it isextremely hard to localize the origin of error.

[0011] If the light signal propagating in an optical transport fibercould be analyzed in an accurate and simple fashion, without interferingwith the signal, any flaws in the optical communications network couldbe detected at an early stage. In the prior art, such analysis has beentoo complicated, too expensive and too inaccurate to gain widespreadacceptance.

[0012] Obviously, it is of utmost importance to be able to monitor thepower spectrum of the light propagating in the optical transport fiber.

[0013] It is known in the prior art to use a phase grating in an opticalfiber in order to filter out one desired wavelength from a broadbandlight signal. A phase grating will reflect one predefined wavelength andleave other wavelengths essentially undisturbed.

[0014] An optical phase grating is a structure of essentiallyperiodically varying refractive index in an optically transparentmedium. An overview of the technology is given in M. C. Hutley,“Diffraction gratings”, Academic Press, London (1982). When light isincident on an optical phase grating, a small part of the incident lightis reflected off each grating element (period). When a multiplicity ofgrating elements are arranged after each other (i.e., arranged as aphase grating), the total reflected light would be the sum of all thesesingle reflections. The fraction of the incident light reflected offeach grating element is determined by the depth (amplitude) of therefractive index modulation in the phase grating. The deeper themodulation, the larger fraction of the incident light is reflected offeach grating element. If the incident light is essentially normal to thegrating (i.e., to the grating elements), the grating is said to act inthe Bragg domain and is named “Bragg grating”. Light reflected off eachgrating element will thus overlap the light reflected off the otherelements, causing interference. For a certain wavelength, all thesereflections are in phase, whereby constructive interference is effected.Although each reflection is small, a substantial reflection is obtaineddue to constructive interference. The wavelength for which constructiveinterference is effected is called the “Bragg wavelength,” λ_(Bragg),and is given by (at normal angle of incidence)

λ_(bragg)=2nΛ

[0015] where n is the average of the refractive index, and Λ is thegrating period.

[0016] If the grating period, Λ, varies along the grating, the gratingis said to be a “chirped” Bragg grating. In a chirped Bragg grating,different wavelengths are reflected in different portions of thegrating, in fact, making the Bragg grating a broadband reflector, achirped Bragg-reflector.

[0017] U.S. Pat. No. 6,052,179 discloses a system for determining theaverage wavelength of light transmitted through an optical fiber. Inthis system, a chirped Bragg grating is provided in an optical fiber,the grating having a modulation amplitude that varies from a first endto a second end of the fiber grating. Thus, the reflectivity at onewavelength is different from the reflectivity at another wavelength,since different wavelengths are reflected at different portions of thegrating. Based on the output from two photo detectors, one detecting thelight transmitted through the grating and the other acting as areference, the average wavelength of light transmitted through thegrating is determined. However, this approach has several disadvantages.Firstly, the light to be analyzed has to be divided into two separatefibers, which is a serious complication in itself. Secondly, only theaverage wavelength can be determined. There is no possibility for properspectrum analysis by the system disclosed in the above reference.Furthermore, the system is intended for sensing applications, whereexternal influence changes the average wavelength coming into thesystem.

[0018] Previous attempts to alter the reflective properties of anoptical Bragg grating include creation of an acousto-optic superlatticeimposed on a chirped Bragg grating. Such an arrangement is described byChen et al. in “Superchirped moiré grating based on an acousto-opticsuperlattice with a chirped fiber Bragg grating”, OPTICS LETTERS, Vol.24, No. 22, pp. 1558-1560. According to this reference, multipletransmission peaks are obtained by superimposing an acoustic wave on achirped Bragg grating. The spacing of the transmission peaks is variedby the acoustic frequency. However, this arrangement does not permittransmission of one single wavelength only, and the variation of thespacing of the transmission peaks by acoustic frequency is very limited.

SUMMARY OF THE INVENTION

[0019] The present invention provides new methods and arrangements forestablishing transmission of light through a reflecting Bragg grating,and for utilizing such transmission for analysis of the characteristicsof a light signal. The drawbacks and limitations associated with theprior art are effectively eliminated by a method and an arrangement ofthe general kind set forth in the accompanying claims.

[0020] The present invention has further advantages, which will beapparent from the detailed description set forth below.

[0021] It is a general object of the present invention to provide amethod of establishing transmission of light through a broadband,chirped Bragg-reflector. The method can be used for analysis of thepower spectrum of a light signal, as well as for other applicationswhere it is desirable to transmit a certain wavelength component oflight through a structure at a certain instant in time.

[0022] Furthermore, it is an object of the present invention to providea method and an arrangement for spectrum analysis of a light signal,which method and arrangement essentially eliminate the aforementioneddrawbacks and limitations of the prior art. Briefly stated, this isobtained by establishing transmission of light through a chirpedBragg-reflector by means of a longitudinal acoustic pulse being presentin the grating structure. The presence of the acoustic pulse alters thereflective properties of the Bragg-reflector, thus giving rise totransmission of a specific wavelength of light for each position of theacoustic pulse in the Bragg-reflector.

[0023] According to a first aspect of the present invention, a method ofestablishing transmission of light through a chirped Bragg-reflector isprovided, by which method a certain wavelength component is transmittedthrough the Bragg-reflector at a certain (corresponding) instant intime. In an unperturbed state, the Bragg-reflector is reflectingessentially all incident light within a predefined wavelength range.According to the invention, light is incident into an optical waveguideincorporating a chirped Bragg-reflector. The reflective properties ofsaid Bragg-reflector are altered by sending a longitudinal acousticpulse into said waveguide for propagation along the same. For eachlocation of said acoustic pulse in the chirped Bragg-reflector, thereflectivity for a wavelength associated with said location in saidBragg-reflector is altered, thereby establishing transmission of thewavelength at issue.

[0024] According to a second aspect of the present invention, a newmethod of analyzing the power spectrum of a light signal is provided.The method of analyzing the power spectrum is based on theaforementioned method of establishing transmission of light through achirped Bragg-reflector. Briefly stated, analysis of the power spectrumis obtained by monitoring the light transmitted through theBragg-reflector, and by subsequent analysis of the monitored signal.

[0025] One advantage of this method is that it is sufficient to tap offonly a small part (typically about 1% or less) of the light propagatingin an optical transport fiber into a secondary fiber. The light in thesecondary fiber is then analyzed, and the interfering effect on thetransport fiber is negligible. The light is tapped off from thetransport fiber by non-wavelength discriminating coupling means, asknown in the art. By analyzing the light in the secondary fiber by themethod according to the present invention, the power spectrum of thelight signal propagating in said transport fiber is determined with veryhigh accuracy. Furthermore, the light signal in the transport fiber isessentially undisturbed, apart from the 1% tapped off.

[0026] Another advantage of the present invention is that it also allowsfor real time supervision of a fiber-based communications system. Theinformation obtained from such supervision can advantageously beutilized for controlling other equipment connected to said system, suchas amplifiers, filters, etc. In-line amplifiers and filters can becontrolled in a feedback configuration with a spectrum analyzeraccording to the present invention.

[0027] Yet another advantage of the present invention is that awavelength scan of transmitted light is obtained, which is veryconvenient for spectrum analysis. The present invention can provide forrepeated wavelength scans, thereby facilitating interpretation andanalysis of the monitored signal.

[0028] According to a third aspect of the present invention, anarrangement is provided for measuring and characterizing the powerspectrum of a light signal propagating in an optical fiber. Such anarrangement includes a chirped Bragg-reflector operating in accordancewith the methods above.

[0029] The present invention is based on the general insight that thereflective properties of a chirped Bragg grating (or chirpedBragg-reflector) can be altered by a longitudinal acoustic pulsepropagating along the grating. More particularly, the present inventionis further based on the deeper insight that a longitudinal acousticpulse of carefully chosen shape propagating along the grating can givehighly accurate, and wavelength separated in time, transmission throughthe chirped Bragg grating.

[0030] According to one embodiment of the present invention, a method ofestablishing transmission of light through a chirped Bragg-reflectorincludes the steps of directing light into a light-guiding structureprovided with said Bragg-reflector, and sending a longitudinal acousticpulse along said light-guiding structure, thereby locally andtemporarily altering the reflective properties of the Bragg-reflectorfor a certain wavelength corresponding to the momentary position of thetravelling acoustic pulse.

[0031] According to another embodiment of the present invention, anoptical fiber is provided with a chirped Bragg grating. The fiber has aninput end and an output end, for the input of a light signal into saidfiber and for the output of a transmitted part of said light signal,respectively. Connected to the optical fiber is an acoustic actuator forthe emission of a longitudinal acoustic pulse into said fiber. Theacoustic pulse is given a shape that has the effect of lowering thereflectance of the chirped Bragg-reflector for a certain wavelength at acertain instant in time. Preferably, the acoustic pulse is given ananti-symmetric shape, whereby an etalon effect is achieved in theproximity of the travelling acoustic pulse, thereby locally lowering thereflectivity of the chirped Bragg-reflector to essentially zero.

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The following detailed description of preferred embodiments isbetter understood when read in conjunction with the accompanyingschematic drawings, in which:

[0033]FIG. 1 is a schematic outline of the basic principle behind thepresent invention,

[0034]FIG. 2 shows a typical transmittance profile of a chirpedBragg-reflector in an unperturbed state,

[0035]FIG. 3 illustrates a transmittance peak caused by the presence ofan acoustic pulse in the chirped Bragg-reflector that sweeps through thereflectance band of the Bragg-reflector,

[0036]FIG. 4 is an enlarged view of the transmittance peak caused by thepresence of an acoustic pulse in the Bragg-reflector,

[0037]FIG. 5 shows an overview of an arrangement for spectrum analysisaccording to the present invention,

[0038]FIG. 6 shows the acoustic actuator and the fiber arrangementaccording to the present invention,

[0039] FIGS. 7-9 shows the attachment of the fiber to the acousticactuator in greater detail,

[0040]FIG. 10 shows the shape of an acoustic pulse in accordance withthe present invention, and

[0041]FIG. 11 shows a detector output signal and illustrates acorrelation interpretation method according to the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0042] Typically, and in accordance with one embodiment of the presentinvention, a chirped Bragg-reflector is provided in an optical fiber. Asdescribed above, the chirped Bragg-reflector is a distributed structurethat reflects light within a predefined wavelength range. The wavelengthreflected in each portion of the distributed structure depends on thelocal period of the grating constituting the Bragg-reflector. Thegrating period is defined as the distance between two adjacent gratingelements. The shorter the grating period, the shorter the wavelengththat is reflected.

[0043] In a chirped grating, the grating period varies along thegrating, usually in a steady fashion (e.g., from a short period to alonger period, or from a long period to a shorter period). The periodcan vary continuously or stepwise. In a portion of the grating (i.e.,the chirped Bragg-reflector) where the grating period is short, a shortwavelength is reflected, and in a portion of the grating where thegrating period is long, a long wavelength is reflected. In effect,different wavelengths are reflected in different portions of the chirpedBragg-reflector.

[0044] Compression and/or elongation of the fiber can alter thereflective properties of the chirped Bragg-reflector. According to thepresent invention, this is accomplished by means of a longitudinalacoustic pulse which causes temporary compression and/or elongation ofthe fiber as it propagates along the fiber. In fact, such compression orelongation introduces both refractive index changes and actual pathchanges, the latter, however, being predominant. The aggregate effect onlight is a combination of the change in refractive index and the changeof the actual period of the grating in the fiber. From aphenomenological point of view, the crucial quantity is the optical pathtraversed by the light in the fiber. The optical path is defined as theactual, physical path multiplied by the refractive index of the mediumin which the light is propagating. For example, an increase of theeffective grating period in a Bragg grating in an optical fiber could beobtained either by increasing the refractive index of the fiber, or byincreasing the physical length of the period (i.e., the fiber).

[0045] When stretching or compressing an optical fiber provided with aBragg-reflector, the change in refractive index is negligible comparedto the actual change in physical path. Therefore, if any portion of achirped Bragg-reflector is stretched, this portion will reflect a longerwavelength than it would do in its unperturbed state. This actuallyapplies for any Bragg-reflector, even with a constant grating period.

[0046] The basic principle behind the present invention will now bedescribed in greater detail with reference to FIG. 1.

[0047] An optical fiber 10 incorporates, in the core 102 thereof, achirped Bragg grating 104. The Bragg grating is incorporated into theoptical fiber by means of methods known in the art. The fiber 10 has aninput end 11 and an output end 12, for receiving input of a light signaland for delivering an output signal, respectively. The chirped Bragggrating exhibits a modulation that is strong enough to reflectessentially all incident light within a predefined wavelength range whenthe grating is in an unperturbed state. Due to this fact, the chirpedBragg grating is referred to as a chirped “Bragg-reflector.”In thiscase, it is assumed that the Bragg-reflector reflects all lightwavelengths between λ₀ and λ_(k). Any wavelength of light within thisrange will be reflected if sent into the optical fiber incorporatingsaid Bragg-reflector.

[0048] In order to establish transmission of light within thiswavelength range (λ₀ to λ_(k)) through the chirped Bragg-reflector, alight signal is first directed into the input end 11 of the opticalfiber 10, as is indicated by the arrow 111. If the optical fiber, andthus the chirped Bragg-reflector, is in an unperturbed state, the lightwill be reflected back towards the input end 11, and no light will betransmitted to the output end 12. A longitudinal acoustic pulse 120 isthen sent along the optical fiber. In this case, and as shown in thefigure, the longitudinal acoustic pulse 120 has an anti-symmetric shape,thereby causing in one part a localized and temporary elongation of theoptical fiber 10 and, in another part, a localized and temporarycompression of the optical fiber 10. The acoustic pulse 120 alters thereflective properties of the chirped Bragg-reflector 104, andeffectively establishes transmission of light through the optical fiber10.

[0049] In FIG. 1, the chirped Bragg-reflector is shown to have a gratingperiod that decreases from left to right. This means, as describedabove, that a longer wavelength is reflected in the left portion of theBragg-reflector than in the right portion of the Bragg-reflector. Whenthe longitudinal acoustic pulse travels along the grating from left toright, and hence locally and temporarily alters the reflectiveproperties of the reflector, longer wavelengths will be transmittedfirst, followed by shorter wavelengths.

[0050] The mechanism behind the alteration of the reflective propertiesof the chirped Bragg-reflector will now be described in more detail.

[0051] The acoustic pulse that is sent along the optical fiber has oneelongating part causing a temporary elongation of the optical fiber, andone compressing part causing a temporary compression of the opticalfiber, as mentioned above. In the present example, the acoustic pulse isessentially anti-symmetric, each of said parts being equally important.When the acoustic pulse is sent along the chirped Bragg-reflector (theoptical fiber) in a direction in which the grating exhibits a decreasinggrating period, it is essential that the acoustic pulse has a leadingportion that causes a decrease of the grating period (i.e., a portionthat causes a compression of the optical fiber) and a trailing portionthat causes an increase of the grating period (i.e., a portion thatcauses an elongation of the optical fiber). The leading portion of theacoustic pulse will then temporarily alter the local grating period suchthat the altered local grating period actually gives a reflection for asomewhat shorter wavelength than in its unperturbed state. The trailingportion of the acoustic pulse will, at the same time, temporarily alterthe local grating period such that the altered local grating periodactually gives a reflection for a somewhat longer wavelength than in itsunperturbed state. Effectively, a window is created where there isreduced or no reflection, since the grating period of the chirpedBragg-reflector is altered in such a way that there is no grating periodgiving reflection for a certain wavelength. As the longitudinal acousticpulse propagates along the chirped Bragg-reflector, the above situationwill successively apply for shorter wavelengths, thereby creating awavelength scan of transmitted light.

[0052] If the acoustic pulse propagates along the chirpedBragg-reflector in a direction with increasing grating period, theacoustic pulse should, of course, have the opposite phase in order tocause a similar effect as before.

[0053] When the acoustic pulse is given an anti-symmetric shape with asufficiently short rise time (or fall time) between the compressive andthe elongating portion, an etalon effect is obtained, where thereflectivity for a certain wavelength drops essentially to zero.

[0054] In the lower part of FIG. 1, the acoustic pulse is shown in acoordinate system at two different instants. The compression/elongationis illustrated as a change in longitudinal dimension Δz at a first pointin time t₁ and a second point in time t₂. At time t₁, the acoustic pulseis in a first portion of the Bragg-reflector where the unperturbedgrating period is large, hence causing transmission of a long wavelengthλ₁, through the optical fiber. At a later time t₂, the acoustic pulsehas propagated to a second portion of the Bragg-reflector where theunperturbed grating period is shorter than in the first portion, hencecausing transmission of a shorter wavelength λ₂ through the opticalfiber. Consequently, the output power at the output end 12 of theoptical fiber at a certain point in time is associated with the power ofa corresponding wavelength. In this example, the output power at timet₁, which is denoted P(t₁), is associated with the power of the incidentlight at wavelength λ₁, which is denoted P(λ₁). Thus, P(t₁)=P(λ₁),P(t₂)=P(λ₂), etc., any losses neglected.

[0055] In some cases, small leakage through the unperturbedBragg-reflector can be allowed, since compensating for such leakagewould be straightforward.

[0056]FIG. 2 shows schematically the transmittance of theBragg-reflector as a function of wavelength. Illustrated in the figureis a reflection band, where there is essentially no transmission. Thefull width at half maximum (FWHM) of this reflection band is, in theexemplary embodiment, 40 nm. Typically, some 30 nm of the reflectionband exhibits no transmission at all in the unperturbed state of theBragg-reflector, i.e., practically 100 percent reflection. However,almost any width of the reflection band could conceivably be obtained bychoosing the grating period and the chirp accordingly.

[0057]FIG. 3 illustrates a narrow transmittance peak that sweeps throughthe reflection band as the acoustic pulse propagates along theBragg-reflector. In the figure, the transmittance peak is shown to sweepfrom longer wavelengths to shorter (from left to right in the figure, asindicated by the arrow), which is the case when the acoustic pulsepropagates through the chirped Bragg-reflector in a direction from alonger grating period to a shorter grating period. Of course, if theacoustic pulse was propagating in the opposite direction through thechirped Bragg-reflector, the transmittance peak would sweep in theopposite direction, i.e., from a shorter wavelength to a longerwavelength. When the transmittance peak sweeps through the reflectionband of the chirped Bragg-reflector, a wavelength scan of transmittedwavelengths is effectively achieved.

[0058]FIG. 4 is an enlarged illustration of the transmittance peak. Thewidth of the transmittance peak can be chosen by proper selection ofchirp of the Bragg-reflector (i.e., the rate of change of the gratingperiod) and the modulation depth of the grating. The transmittance peakcould be as narrow as 40 pm. However, when the present invention isutilized in a spectrum analyzer in a WDM system, a transmittance peak ofabout 0.1 nm (FWHM) is sufficient in order to separate the differentchannels. As indicated in FIG. 4, the transmittance peak can reach 100percent by choosing the proper system parameters.

[0059]FIG. 5 shows a schematic overview of a preferred embodiment of anarrangement for analyzing the power spectrum of a light signal accordingto the present invention.

[0060] The arrangement is utilized for analysis of the power spectrum ofa light signal propagating in an optical transport fiber. A smallfraction (typically about 1 percent) of the light propagating in theoptical transport fiber is tapped of by a fiber coupler. The lighttapped off is subsequently directed into a fiber provided with a chirpedBragg-reflector. Transmission of light through the chirpedBragg-reflector is achieved in accordance with the inventive methoddescribed above by means of sending an acoustic pulse along the fiber.The light thus transmitted is monitored and analyzed in order to providespectrum analysis of the light signal directed into the fiber.

[0061] The preferred arrangement according to the present invention willnow be described in greater detail with reference to FIG. 5.

[0062] The arrangement is operatively connected to an optical transportfiber 500 through a fiber coupler 510. The coupler 510 is a tap couplertapping off 1 percent of the power of the light propagating in theoptical transport fiber 500. The coupler 510 is non-discriminating interms of wavelength, which means that each wavelength componentpropagating in the fiber 500 is tapped off with the same couplingfactor. Effectively, the power spectrum of the 1 percent tapped off hasthe same profile as the power spectrum of the original light signalpropagating in the transport fiber 500, but at a lower peak power.

[0063] The light tapped off the transport fiber 500 is directed into asecondary fiber 502 and, incident upon a broadband Bragg-reflector 520,incorporated into the fiber 502, which Bragg-reflector operates as abroadband filter for incident light. Only light within the reflectanceband of the filtering reflector 520 is reflected back into the secondaryfiber 502, the broadband Bragg-reflector 520 thus pre-filtering thelight signal to be analyzed. Typically, this pre-filtering of the lightby means of the filtering reflector 520 selects (i.e., filters out byreflection) the signals in the C- or L-band of the telecommunicationsbands. The reflected light is subsequently directed into a portion ofthe secondary fiber incorporating a second Bragg-reflector 550. Thesecond Bragg-reflector 550 is a chirped Bragg-reflector reflectingessentially the same wavelength band as the filtering reflector 520.

[0064] The second Bragg-reflector 550 is utilized for establishingtransmission of light towards a photodetector 556 provided at the outputend of the secondary fiber 502. The transmission of light towards thephotodetector 556 is obtained by sending a longitudinal acoustic pulsealong the portion of the secondary fiber 502 incorporating the chirpedBragg-reflector 550 according to the present invention. By sending alongitudinal acoustic pulse along the chirped Bragg-grating 550,transmission through the secondary optical fiber 502 is achieved in avery precise fashion. The mechanism behind the transmission is describedin detail above.

[0065] The arrangement for analyzing the power spectrum, according tothe present invention, includes an acoustic actuator (not shown in FIG.5) for sending longitudinal acoustic pulses along the chirpedBragg-reflector 550 in the secondary optical fiber 502; a photodetector556 for detecting the power of the light transmitted through theBragg-reflector 550; programmable logic 600, in the form of an FPGA(Field Programmable Gate Array), for storing the waveform of theactuating pulses, and for controlling other hardware and performingsimple signal analysis; a digital to analog converter 610 (DAC) forconverting digitally encoded output from the FPGA 600 into analog form;an analog to digital converter 630 (ADC) for converting monitored analogsignals into digital form for further storing and processing; a memory640 (Mem) for storing measurement data; a delay line 650 (Del) forsub-sampling data in order to improve measurement resolution; a clockcircuit 620 (Clk) for supplying clock pulses to the logic 600; a digitalsynthesizer circuit 622 (DDS) for tuning the clock frequency and therebytuning the repetition rate of actuating pulses to the acoustic actuator;and a processing unit 660 (CPU) for processing data and forcommunicating with the logic 600 and other peripherals.

[0066] The longitudinal acoustic pulse to be sent along the chirpedBragg-reflector 550 is excited by the acoustic actuator (not shown inFIG. 5, but shown in great detail in FIGS. 6-9) attached to thesecondary fiber 502. The supply of actuating pulses to the acousticactuator is controlled by the programmable logic 600 (FPGA). In the FPGA600, the waveform of the actuating pulses is stored in digital form. Afirst output 601 of the FPGA 600 is connected to a first input of adigital to analog converter 610 (DAC). The DAC is operative to convertthe digitally encoded waveform of the actuating pulses to an analogwaveform and to send the analog output signal to an amplifier 612. Theamplified output of the amplifier 612 is fed to the acoustic actuator asactuating pulses. A second output 602 of the FPGA 600 is connected tothe amplifier 612 in order to provide control signals from the FPGA 600to the amplifier 612.

[0067] The FPGA 600 is triggered by clock pulses from a clock circuit620 (Clk). The output of the clock circuit 620 is operatively connectedto the FPGA 600 via the digital synthesizer circuit 622 (DDS). Thedigital synthesizer circuit 622 is operative to tune the frequency ofthe clock circuit 620 in order to obtain tuning capability of theactuating pulses to the acoustic actuator. In order to control the DDS622, a third output 603 of the FPGA 600 is connected to the DDS. Theclock circuit 620 is also operatively connected, via said DDS, to theDAC 610 and to the delay line 650 and the ADC 630.

[0068] The photodetector 556 is operative to detect the power of theoutput signal from the secondary fiber 502, after transmission throughthe chirped Bragg-reflector 550. As described above, the output power ateach instant corresponds to the power of a certain wavelength componentin the light signal directed into the secondary fiber 502. The lightdirected into the secondary fiber, in turn, corresponds on a one-to-onebasis to the light propagating in the optical transport fiber 500. Thephotodetector 556 is arranged to provide a detector output signal,associated with the power of the detected signal, to a preamplifier 632.The output signal from the preamplifier 632 is passed to the ADC 630.The ADC 630 is operative to convert the amplified detector output signalinto digitally encoded form and to provide the digitally encodedmeasurement data to the memory 640 for storing. The delay line 650 andthe amplifier 632 (and thus the ADC 630) are controlled by the FPGA 600through fourth 604 and fifth 605 outputs of the FPGA, respectively.

[0069] The contents of the memory 640 is readable to the programmablelogic 600. The programmable logic 600 is arranged to read the storedmeasurement data from the memory 640 and to perform simple signalanalysis of the stored data. The analysis performed by the programmablelogic 600 is the basis for the control that the logic 600 exerts on theDDS 622, the delay line 650, as well as for the actuating pulses fed tothe DAC 610. Furthermore, the programmable logic 600 can communicatewith a processing unit 660 (CPU) in order to transfer measurement datafor further processing and supervision of system performance.

[0070] The processing unit 660 has a memory 662 for storing bothprocessed and non-processed data. Also, the processing unit hasconnections 664 for communicating with other peripherals (not shown),such as a display device, a keyboard, an I/O-device for communicatingwith other apparatus, etc.

[0071] Furthermore, the output of the filtering reflector 520 canadvantageously be connected to a second arrangement according to thepresent invention. For example, if the C-band of the communicationsbands is filtered out by a first filtering reflector and analyzed in afirst arrangement according to the present invention, then a secondarrangement can be coupled to the first for analyzing the power spectrumof light signals in the L-band of the communications bands. A personhaving ordinary skill in the art will find it straightforward to combinetwo arrangements according to the present invention, in order to analyzetwo different communications bands simultaneously.

[0072] The acoustic actuator and the supply of longitudinal acousticpulses to the portion of the secondary optical fiber 502 incorporatingthe chirped Bragg-reflector 550 will now be described in more detailwith reference to FIGS. 6-9.

[0073]FIG. 6 shows schematically an acoustic actuator module 700 and thesecondary fiber 502 attached thereto.

[0074] The acoustic actuator comprises a piezo device 705, made from apiezo-electric material, arranged in a rod 706, 708 capable of guidingacoustic pulses along its longitudinal axis. The rod is made fromaluminium, but could also be made from, for example, copper or othersuitable material. The piezo device 705 has two connectors attached toits surface for the supply of actuating pulses thereto. The piezo deviceis responsive to actuating pulses by changing its longitudinal size.Hence, when a voltage is applied to the piezo device 705, the size ofthe device changes accordingly. If properly actuated, the acousticactuator emits an acoustic pulse into the rod 706, 708 (as indicated inthe figure by the four solid arrows), the rod guiding said pulse towardsits ends. One end 706 of the rod is provided with an acoustic attenuator707, effectively damping the acoustic pulse to essentially zeroamplitude. The other end 708 of the rod is firmly attached to the fiber502, thereby ensuring that the acoustic pulse, emitted by the acousticactuator, is transferred to the fiber 502. When the acoustic pulsereaches the fiber end 709 of the rod 708, some of the acoustic power isreflected back towards the piezo device 705, and some of the acousticpower is transferred to the fiber 502. The part of the acoustic powerthat is reflected back into the rod passes through the piezo device 705and is effectively damped by the acoustic attenuator 707. This ensuresthat only one pulse is sent into the fiber 502 by each actuation of thepiezo device 705.

[0075] The acoustic pulse propagates along the fiber 502, and thus alongthe chirped Bragg-reflector 550, thereby establishing transmission oflight through the Bragg-reflector 550 and effectively causing awavelength scan of wavelengths transmitted towards the detector 556.

[0076] At the output end of the secondary fiber 502, the fiber isclamped by a clamping block 702. The clamping block 702 forms a firstreflection point 710 and is arranged to give a reflection of theacoustic pulse back towards the rod 708 and the acoustic actuator 705.Furthermore, when the reflected acoustic pulse reaches the attachment ofthe fiber to the actuator, the pulse is again reflected, this time backtowards the output end of the fiber, the attachment of the fiber to theactuator thereby forming a second reflection point 720 for the acousticpulse.

[0077] It is preferred that acoustic pulses are sent into the fiberrepeatedly, in order to provide repeated scans of transmittedwavelengths. In this embodiment, an acoustic pulse is sent into thefiber 502 at the time when the reflection in the fiber of a previouspulse reaches the attachment point between the rod and the fiber. Theacoustic pulse propagating back and forth between the reflection points710, 720 thereby achieves some additional amplitude each time the pulsereaches the reflection point at the actuator 720. Preferably, thereflection of the acoustic pulse at each reflection point 710, 720exhibits some predefined loss. This ensures that the acoustic pulseeventually dies away in a known manner. Another advantage achieved bysuch a loss is that the timing requirements on the acoustic pulses fromthe actuator are alleviated to some extent. If an acoustic pulse fromthe actuator is sent into the fiber 502 slightly off schedule, thisunwanted or untimely acoustic pulse will eventually die away, therebypreventing the accumulation of systematic error.

[0078] As described above, it is essential that the phase of theacoustic pulse corresponds to the chirp of the Bragg-reflector in orderto give the desired result. Conveniently, the reflection of the acousticpulse at each reflection point 710, 720 introduces a phase change of 180degrees into the acoustic pulse. The phase of the acoustic pulse willconsequently be suitable for establishing the desired transmissionthrough the Bragg-reflector when the acoustic pulse is propagating backtowards the actuator, as well. This dual-pass feature of the presentinvention can conveniently be utilized for correlation evaluation inorder to increase resolution and/or detect and correct any possibleflaws.

[0079] The dual-pass feature is also utilized for the actualinterpretation of the detected light signals. In order to ascribe acertain wavelength to a certain instant, i.e., to an output signal fromthe detector at a certain time, a correlation procedure is utilized. Afirst pass of the acoustic pulse through the chirped Bragg-reflectorcauses transmission of a first spectrum profile. A second pass of theacoustic pulse through the chirped Bragg-reflector, now in the oppositedirection, causes transmission of a second spectrum profile, which isessentially the same spectrum profile as the first spectrum profile, butin reversed timely order, since the acoustic pulse now propagates in theopposite direction through the Bragg-reflector. The acoustic pulse isthen reflected back towards the Bragg-reflector a third time, causingtransmission of a third spectrum profile. The third spectrum profile isessentially identical to the first spectrum profile if the first and thethird pass of the acoustic pulse through the chirped Bragg-reflector aresufficiently close in time (since the light signal to be analyzed hasnot had time to change significantly between the first and the thirdpass).

[0080] Now, a correlation between the first spectrum profile and thethird spectrum profile provides a first time interval, corresponding tothe passage time of the acoustic pulse to complete one round trip in thefiber between the two reflection points. This first time interval shouldbe constant, unless the length of the fiber is changed by, for example,a temperature change (i.e., thermal expansion or contraction of thefiber).

[0081] In order to correlate the second spectrum profile and the firstspectrum profile, the second profile must first be mirrored. Mirroringof the second spectrum profile about a reference wavelength, andsubsequent correlation between the mirrored spectrum and the firstspectrum, provides a second time interval, corresponding to the timeelapsed between one transmission of said reference wavelength, and thefollowing transmission of the same reference wavelength during thefollowing passage of the acoustic pulse through the chirpedBragg-reflector.

[0082] The relationship between the first time interval and the secondtime interval is constant, even if the length of the fiber changes dueto, for example, temperature changes. This means that there is a uniquenumber (equal to the relationship between the first and the second timeintervals) associated with each reference wavelength. By measuring thetime intervals, calculating the relationship, and comparing therelationship with a stored value in a look-up table, the correspondingreal value for each reference wavelength is obtained.

[0083] The analysis of the spectrum profiles is preferably done aftersampling the detector output signal into a digitally encoded form, eachtime interval thus corresponding to a certain number of samples.

[0084] Furthermore, by monitoring said first time interval, a change infiber length due to, for example, temperature changes, can be detected.Consequently, the repetition rate of the acoustic pulses can becontrolled to comply with the length of the fiber. Hence, resonantfeeding of acoustic pulses from the actuator is possible even if thelength of the fiber is changed.

[0085] By basic symmetry considerations, it is obvious that the lengthof the piezo device 705 must be carefully selected to suit the desiredpulse length of the acoustic pulse to be sent into the fiber 502. Aperson having ordinary skill in the art will find the suitable length ofthe piezo device by simple tests and/or computer simulations.

[0086] Having described the general features of the acoustic actuator,the attachment of the fiber 502 thereto will now be described in moredetail with reference to FIGS. 7-9.

[0087]FIG. 7 shows schematically the end 708 of the actuator rod towhich the fiber 502 is attached. As illustrated in the figure, the endof the rod is provided with a shoulder 718, which the fiber 502 is tobear against.

[0088] In FIG. 8, the end of the rod is shown in greater detail. Thefiber 502 bears against the shoulder 718 and is fixed thereto by meansof a contact block 728. The contact block 728 is tightened to the fiber502 by means of a wire 730, in this case a tungsten wire, surroundingthe entire rod 708. The purpose of the contact block 728 is todistribute the force from the wire 730, and to give a well-definedinteraction length L between the fiber 502 and the actuator rod.

[0089] The placement and the length of the shoulder 718 requires someconsideration. The placement of the shoulder 718 needs to be close tothe end of the actuator rod which, in practice, means that the center ofthe shoulder should be within a fraction of the shortest wavelength (thehighest frequency) building up the acoustic pulse. Such an arrangementof the shoulder minimizes distortion of the acoustic pulse whentransferred from the rod to the fiber. The placement of the shoulder 718can also be utilized to obtain a desired distortion, e.g., a low passfiltering, of the acoustic pulse when it enters the optical fiber 502.

[0090]FIG. 9 is a cross-sectional view of the attachment of the fiber502 to the actuator rod 708. The fiber 502 is clamped to the rod 708 bymeans of the contact block 728, and fixed into place by the wire 730.The wire 730 is wound one full turn around the rod 708, and is tightenedby a spring 732, as shown. This arrangement ensures firm attachment ofthe fiber 502 to the actuator rod 708 in a well-defined way.

[0091] Thus, the proximity of the attachment point (i.e., the shoulder718) to the end of the actuator rod 708 allows for reliable transfer ofacoustic pulses into the fiber 502. More particularly, the closeness ofthe shoulder 718 to the end of the rod eliminates any influence frompossible interference between a portion of the acoustic pulsepropagating in one direction and a portion of the acoustic pulsepropagating, after reflection, in the opposite direction.

[0092]FIG. 10 shows an example of a longitudinal acoustic pulse,illustrating its shape as it propagates in the actuator rod 708 and inthe optical fiber 502 according to the present invention. In the shownexample, the pulse is anti-symmetric and plotted in terms of change inlength Δz. The shape shown is preferred, since such a shape gives atransmittance peak of close to 100 percent transmission of a certainwavelength.

[0093] Although the present invention has been described with referenceto the drawings and by way of preferred embodiments, it is to beunderstood that the embodiments described can undergo severalalterations and modifications without departing from the scope of theinvention as claimed in the accompanying claims.

What is claimed is:
 1. A method of establishing transmission of light,comprising the steps of: providing a light-guiding structureincorporating a chirped Bragg-reflector, directing light into an inputend of said light-guiding structure, and sending a longitudinal acousticpulse into said light-guiding structure for propagation along the same,the acoustic pulse being such that, for each location of said acousticpulse in said Bragg-reflector, the reflectivity for a wavelengthassociated with said location in said Bragg-reflector is altered.
 2. Themethod of claim 1, wherein the propagation of the acoustic pulse alongsaid light-guiding structure causes an alteration of a local gratingperiod of said chirped Bragg-reflector to provide, for each location ofsaid acoustic pulse in said Bragg-reflector, transmission of awavelength associated with said location, thereby providing a wavelengthscan of transmitted wavelengths as the acoustic pulse propagates alongsaid Bragg-reflector.
 3. The method of claim 1, wherein the acousticpulse has a first part and a second part, said first part causing atemporary increase of the local grating period of the chirpedBragg-reflector, and said second part causing a temporary decrease ofthe local grating period of said Bragg-reflector.
 4. The method of claim3, wherein the portion of said chirped Bragg-reflector containing thefirst part of said acoustic pulse has, in its unperturbed state, alonger grating period than the portion of said Bragg-reflectorcontaining the second part of said acoustic pulse.
 5. The method ofclaim 1, wherein the step of sending a longitudinal acoustic pulsethrough the light-guiding structure is performed repeatedly, whereby aplurality of acoustic pulses is sent through said light-guidingstructure, of which plurality of acoustic pulses only one acoustic pulseis present in the chirped Bragg-reflector at any one instant.
 6. Themethod of claim 1, further comprising the step of detecting, at anoutput end of the light-guiding structure, the intensity of the lighttransmitted, the light detected at a certain instant in timecorresponding to a certain wavelength, thereby allowing analysis of thepower spectrum of the light directed into said light-guiding structure.7. The method of claim 6, further including the step of prefiltering thelight to be directed into the light-guiding structure by means of abroadband filter, said filter passing essentially the same wavelengthrange as the chirped Bragg-reflector reflects.
 8. The method of claim 3,wherein either one of the first and the second part of the acousticpulse is predominant, the acoustic pulse thereby causing mainly anincrease or mainly a decrease of the local grating period of theBragg-reflector.
 9. The method of claim 1, wherein said light-guidingstructure is the core of an optical fiber.
 10. An arrangement foranalyzing the power spectrum of a light signal, comprising: alight-guiding structure, having an input end and an output end; achirped Bragg-reflector in said light-guiding structure, which chirpedBragg-reflector is provided between the input end and the output end; anacoustic actuator connected to said light-guiding structure; and adetector provided at the output end of said light-guiding structure,said detector delivering a detector output signal, wherein the input endof said light-guiding structure is arranged to receive the light signalto be analyzed, said acoustic actuator is operative to emit alongitudinal acoustic pulse for propagation along the light-guidingstructure, and said detector is operative to detect the light thustransmitted through said Bragg-reflector and to provide a detectoroutput signal associated with the power spectrum of said light signal.11. The arrangement of claim 10, further including a broadband filterprovided in the light path prior to the input end of the light-guidingstructure, said filter being arranged to pass only light of a desiredwavelength range to said light-guiding structure.
 12. The arrangement ofclaim 11, wherein the broadband filter is arranged to pass essentiallythe same wavelength range as the chirped Bragg-reflector reflects. 13.The arrangement of claim 10, further comprising a logic unit connectedto said acoustic actuator, for supplying actuating pulses to saidacoustic actuator.
 14. The arrangement of claim 13, further comprising aprocessing unit, which is arranged to control the operation of the logicunit.
 15. The arrangement of claim 13, wherein the logic unit isarranged to perform basic signal analysis of the detector output signal.16. The arrangement of claim 15, wherein the basic signal analysisperformed by the logic unit is utilized for the control of the acousticactuator.
 17. The arrangement of claim 10, wherein the light-guidingstructure is clamped at a first point and a second point thereof,thereby defining a first and a second reflection point, at whichreflection points the acoustic pulse is reflected, and effectivelyproviding multiple passages of the acoustic pulse through theBragg-reflector.
 18. The arrangement of claim 17, wherein the acousticactuator is connected to the light-guiding structure at either one ofthe first and the second reflection points.
 19. The arrangement of claim18, wherein the acoustic actuator is arranged to emit a longitudinalacoustic pulse into the light-guiding structure at the time when aprevious acoustic pulse, after its reflection, arrives at the pointwhere said actuator is connected to the light-guiding structure.
 20. Amethod of analyzing the power spectrum of a light signal, comprising thesteps of: directing the light signal to be analyzed into a light-guidingstructure incorporating a chirped Bragg-reflector; sending alongitudinal acoustic pulse into said light-guiding structure forpropagation along the same, the acoustic pulse being such that, for eachlocation of said acoustic pulse in said Bragg-reflector, thereflectivity for a wavelength associated with said location in saidBragg-reflector is altered; monitoring the light thus transmittedthrough said light-guiding structure; and analyzing the monitored light,in order to obtain a power spectrum analysis of the light signaldirected into the light-guiding structure.
 21. The method of claim 20,wherein the propagation of the acoustic pulse along said light-guidingstructure causes an alteration of the local grating period of saidchirped Bragg-reflector to provide, for each location of said acousticpulse in said Bragg-reflector, transmission of a wavelength associatedwith said location, thereby providing a wavelength scan of transmittedwavelengths as the acoustic pulse propagates along said Bragg-reflector.22. The method of claim 20, wherein the acoustic pulse has a first partand a second part, said first part causing a temporary increase of thelocal grating period of the Bragg-reflector, and said second partcausing a temporary decrease of the local grating period of saidBragg-reflector.
 23. The method of claim 22, wherein the portion of saidchirped Bragg-reflector containing the first part of said acoustic pulsehas, in its unperturbed state, a longer grating period than the portionof said Bragg-reflector containing the second part of said acousticpulse.
 24. The method of claim 20, wherein the step of monitoring thelight transmitted through the light-guiding structure includes detectingsaid transmitted light by means of a detector, the detector providing adetector output signal associated with the power of the transmittedlight signal.
 25. The method of claim 20, wherein said light-guidingstructure is the core of an optical fiber.
 26. The method of claim 24,wherein the step of sending a longitudinal acoustic pulse along thelight-guiding structure is performed repeatedly, whereby a plurality ofacoustic pulses is sent along said light-guiding structure, of whichplurality of acoustic pulses only one acoustic pulse is present in thechirped Bragg-reflector at any one instant.
 27. The method of claim 26,wherein each acoustic pulse is reflected back and forth between tworeflection points in the light-guiding structure, each successive pulseof said plurality of acoustic pulses being sent into the light-guidingstructure at the time when the previous acoustic pulse, after itsreflection, reaches the acoustic actuator.
 28. The method of claim 27,wherein the wavelength scan obtained by a passage of an acoustic pulsethrough the chirped Bragg-reflector is utilized together with asubsequent wavelength scan for spectrum analysis of the light signaldirected into the light-guiding structure.
 29. The method of claim 28,wherein correlation between different wavelength scans in the detectoroutput signal is performed in order to interpret said detector outputsignals, to ascribe the detector output at a certain instant to aspecific wavelength.
 30. The method of claim 20, further including thestep of pre-filtering the light to be directed into the light-guidingstructure by means of a broadband filter, said filter passingessentially the same wavelength range as the chirped Bragg-reflectorreflects.
 31. The method of claim 24, wherein the detector output signalis fed to a logic unit, said logic unit processing said detector outputsignal to provide a logic unit output signal indicative of the powerspectrum of the light signal directed into the light-guiding structure.